This invention relates to the acid hydrolysis of starch derived from dry milled cereal grains such as corn and milo to provide fermentable sugar.
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity consumed and versatility in product synthesis.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentation of sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth (see, for example, U.S. Pat. No. 3,236,740 and the booklet "Industrial Alcohol by Continuous Fermentation and Vacuum Distillation With Low Energy Consumption", of Chemapec, Inc., Woodbury, N.Y.). For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of energy and raw materials as possible so as to maximize the energy return for the amount of ethanol and enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials. To date, however, relatively little concern has been given to the energy and raw material requirements for manufacturing ethanol from biomass and consequently, little effort has been made to minimize the thermal expenditure and waste incurred in carrying out any of the aforesaid discrete operations involved in the manufacture of ethanol from vegetative sources.
Processes for the acid hydrolysis of carbohydrate polymers, i.e., starch and cellulose, to provide fermentable sugars are known (viz., U.S. Pat. Nos. 2,203,325; 2,210,659; 2,359,763; 2,393,095; 2,395,907; 2,529,131; 2,565,404; 2,946,706; 2,954,304; 2,989,425; 3,169,083; 3,200,012; 3,236,687; 3,313,654; 3,446,664; 3,484,287; 3,607,395; 4,137,094 and 4,155,884). While these and similar processes are for the most part readily adaptable to the hydrolysis of the finely divided, relatively pure starch derived from conventional processes of wet milling cereal grains, their application to the starch-containing fractions obtained from processes of dry milling cereal grains as currently practiced would be uneconomically wasteful of the protein and edible oil associated with these fractions which in the case of corn and milo, is especially significant. Wet milling processes typically remove all but an insignificant amount of non-starch materials, i.e., protein, cellulosic fiber and oil, from the starch component of the grain, the non-starch materials finding valuable application in their own right as animal feeds and feed supplements. However, from the standpoint of producing starch for conversion to sugar, the sugar to dilute ethanol and the dilute ethanol to essentially anhydrous ethanol, conventional wet-milling processes are undesirable because of the need to ultimately remove the large amounts of process water involved.
Where, as in the case of low cost industrial ethanol, a minimal use of energy is necessary to achieve an economically viable process, a relatively energy and capital intensive process such as one based on wet-milled corn starch as the starting material can be disadvantageous. For this reason, the hydrolytic conversion of starch derived from any of the known and conventional dry milling processes is especially desirable in an industrial scale anhydrous ethanol operation since dry milling processes employ no added water beyond the moisture which is already naturally present in the grain. Thus, for example, in a typical dry corn milling process, the kernels are broken by impact and the resulting fractions made up of grits and fine feed which contain the bulk of the starch and significant quantities of oil, protein, and cellulosic fiber, germ which contains most of the oil content of the kernels, and hulls which contain the major portion of the fiber, are separated employing degerminators, sifters, aspirators and gravity separators. A typical dry corn milling product analysis (pounds per bushel) is as follows:
______________________________________ ANALYSIS ON YELLOW CORN #2, LB/100 LB (DRY BASIS) STREAM CORN GERM BRAN GRITS ______________________________________ Starch 34.27 1.33 1.14 31.80 Protein 4.28 0.64 0.27 3.37 Oil 2.05 1.05 0.17 0.83 Fiber 1.22 0.21 0.57 0.44 Nitrogen-Free Extract 5.00 0.21 0.15 0.41 Ash 0.77 1.32 1.50 2.18 Dry Solids 47.59 4.76 3.80 39.03 Moisture 8.41 0.84 0.68 6.89 TOTAL 56.00 5.60 4.48 45.92 ______________________________________
As this analysis indicates, the grits contain 92.8% of the starch, 78.7% of the protein and 40.5% of the oil of the whole corn kernels. Direct complete hydrolysis of the grits would therefore make these substantial amounts of protein and oil unavailable for use as comestibles.
Accordingly, there has heretofore existed a need for a process for converting starch derived from dry milled cereal grains to fermentable sugars while recovering substantially all of the protein and oil content of the starch component of the dry milled grain prior to the complete hydrolysis of the starch. The term "cereal grain" as used herein is to be understood in its commonly used sense and is inclusive of all varieties of corn (maize), milo, wheat, rice, and the like.
In addition to the foregoing consideration, it is known that besides the desired reaction whereby the carbohydrate polymer molecules are split into frementable sugars, other reactions taking place during hydrolysis tend to reduce the maximum theoretical conversion of available carbohydrate to such sugars and produce non-fermentable hydrolysate product. Three of the principal types of undesirable reactions known to take place in acid catalyzed carbohydrate polymer hydrolysis are: degradation (starch molecule is irreversibly destroyed to provide 5-hydroxymethylfurfural which hydrolyzes to levulinic acid and formic acid, and separately to humins); reversion (glucose repolymerizes and/or isomerizes to unfermentable substances); and retrogradation (hydrolysis splits out the branched chain components of the starch molecule leaving a straight chain, lower molecular weight water insoluble polymeric molecule which crystallizes at about 70.degree.-80.degree. C. and becomes resistant to further hydrolysis). In a typical acid hydrolysis process, when equilibrium has been achieved, from about 15 to about 20 weight percent of the depolymerized starch will be present in the form of one or more of the foregoing non-fermentable hydrolysates, the balance of the depolymerized starch being present as glucose and/or other fermentable sugar(s). To the extent non-fermentable products are produced side-by-side with fermentable sugar(s), they represent a loss in yield of the hydrolysis reaction and compromise the usefulness of acid hydrolysis as a process for obtaining fermentable sugar on a large-scale, economical basis.
According to U.S. Pat. No. 2,529,121 referred to supra, the non-fermentable hydrolysate products resulting from one or more of the aforesaid undesirable reactions eventually is recovered in the stillage, or "vinnasse", obtained as a result of the distillation of the dilute ethanol, or "beer", resulting from the fermentation of the fermentable sugar portion of the hydrolysate. To maximize overall ethanol production based on the original quantity of carbohydrate polymer employed, it is proposed in U.S. Pat. No. 2,529,131 to subject the stillage to further acid hydrolysis to convert the unfermentable products therein to fermentable sugars.